chalice 。调用保存后id为空

Question

我已经对此进行了搜索，但仍然无法弄清楚我做错了什么。调用save()后域对象 id是 null .

我已经阅读了如果保存对象时出现问题会发生这种情况，并且 save(flush:true)如果是这种情况，应该会抛出错误，但事实并非如此。看看我的代码和输出:

def pic = new Picture(title:'XX', path:"XXX")
album.addToPictures(pic).save()
if(pic.validate())
   println "no errors. New id: " + pic.id
else
   println "with errors"

输出:

no errors. New id: null

并且在使用 flush:true 时

def pic = new Picture(title:'XX', path:"XXX")
album.addToPictures(pic).save(flush:true)
if(pic.validate())
   println "no errors. New id: " + pic.id
else
   println "with errors"

输出:

no errors. New id: 17

如您所见，创建对象没有任何错误，我应该能够得到 id刚刚调用 save() 后的对象.有任何想法吗？

谢谢

Answer 1

您误解了对象实际持久保存到数据库的时间。调用 obj.save() 时对象不会被持久化，当以下任何一种情况首先发生时，它就会被持久化:

调用 save() 的事务已提交

调用 save() 的 Hibernate session 已关闭

可以使用显式启动事务

SomeDomainClass.withTransaction {
  // code in here runs within a transaction
}

通常，每次调用服务方法时也会隐式启动事务

class MyService {

  void doSomething () {
    // code in here runs within a transaction
  }  
}

如果您不显式或隐式使用事务，则保存的对象会在 Hibernate session 关闭时(大致)在 HTTP 请求完成时持久化。

但是，如果您调用 someObject.save(flush: true)您是在告诉 Hibernate 立即持久化对象，这就是为什么

album.addToPictures(pic).save(flush: true)

为 Picture 分配一个 ID例如，但是

album.addToPictures(pic).save()

仅在封闭 session /事务关闭/提交时分配 ID

更新

进一步你的评论

The problem is that I want to use the id as part of the name of a file I need to save. What about if I get an error saving the file? Should I use a explicit transaction and roll it back?

是的，使用显式事务，并在确定对象已成功持久化后保存文件，如果持久性失败则回滚事务

def pic = new Picture(title:'XX', path:"XXX")

Picture.withTransaction { TransactionStatus status ->        

  try {
    album.addToPictures(pic).save()

  } catch(ex) {
    status.setRollbackOnly()
    throw ex
  }
}

// At this point you can be sure pic has been persisted, so use pic.id to save the file

更新 2

根据您的评论

I don't want to save the file once I'm sure the object has been successfully persisted, but the opposite, I want to persist the object once the file has been successfully saved. So, I'm going to reformulate my questions as "Is there a way to configure Grails so that I can know the id that's going to be assigned to the new object before the object is effectively saved in the database?"

你已经知道了

album.addToPictures(pic).save(flush:true)

将为您提供 Picture 的 ID例如，因此如果您在事务中执行此操作，则无需实际提交事务即可获得 ID。但是，我认为这仅在您使用使用序列的数据库(Oracle，Postgres)时才有效。像下面这样的东西应该工作

Picture.withTransaction { TransactionStatus status ->        

  try {
    def pic = new Picture(title:'XX', path:"XXX")  
    album.addToPictures(pic).save(flush: true)

    // pic.id should now be assigned, so save the file. I'm assuming an
    // an exception will be thrown if saving the file fails

  } catch(ex) {
    // you may also want to try rolling back the file save here, i.e. delete it
    status.setRollbackOnly()
    throw ex
  }
}